Multiply.
Answer: When a number is next to a variable, it means they are multiplied. So, $({1}{y^8})({-2}{y^2})$ is the same as $({1})({y^8})({-2})({y^2})$. Now we can rearrange the factors because multiplication is commutative (a fancy way of saying that the order in which we multiply things doesn't matter). ${(1)(-2)}{(y^8)(y^2)}$ Then simplify, and we're done! ${-2}{y^{10}}$